1. Field of the Invention
This invention relates generally to fault current limiters in power systems, and more particularly the invention relates to apparatus and methods for limiting fault currents in power line transmission and distribution networks by emission control.
2. Description of Prior Art
As electric power utility systems have grown in the past decade, the need has developed for a device to keep the potentially excessively high fault currents within the ratings of existing equipment such as transformers and circuit breakers. To date there is no known commercially available fault current limiter. Known devices have been either technically or economically unsuccessful.
Such devices have fallen into two broad categories. In the first category, a tuned circuit in which the inductive reactance essentially cancels the capacitance reactance is used in series in a power line to give a low impedance at the power frequency. When a fault (i.e. short circuit) occurs, a switch shorts out the capacitor, and the inductive reactance limits the current. Disadvantages include large size, big initial capital costs, and high operating costs.
In the second category, an impedance in parallel with a normally closed bypass switch is placed in series in the power line. When a fault is sensed, the bypass switch is opened and the current is transferred to the current limiting impedance. The approaches tried have included unstable vacuum arc controlled by a magnetic field or other high arcing voltage circuit breakers in parallel with resistors; switches in parallel with fuses in parallel with resistors; and driving superconductors into a highly resistive state. Some of the disadvantages are related to the difficulty in switching and the slow reaction because of the time required for sensing and switching operations.
Some solid state diodes have very low conductivity with rapid current saturation at very low voltage when operated in the reverse bias mode. Putting two such diodes in opposing back-to-back direction in series in a circuit would give current limitation for currents below 1A and less than 10V. However, they would not be practical for power applications requiring currents in kiloamperes and voltages in kilovolts.